Thymus-derived (T) lymphocytes are involved in various cell-mediated immune reactions, such as rejection of foreign grafts, and anti-viral and anti-tumor activity. These lymphocytes are processed within the thymus before emerging as competent T lymphocytes in the peripheral lymphoid organs. Despite extensive work in the past, the mechanism of intro=thymic maturation of thymocytes into competent T lymphocytes is far from clear. Recently it has been indicated by histofluorescence microscopy that thymus and other lymphoid organs receive autonomic nerve fibers and that chemical and surgical sympathectomy alters the immune reactivity of lymphocytes. Since no studies have been done to investigate the innervation of fetal thymus during ontogeny, this investigation is aimed to examine the innervation of fetal thymus from 13 days of gestation to birth by histofluorescence microscopy, silver impregnation techniques and by EM studies. Further, it is also aimed at studying the immunocompetence of thymocytes allowed to develop for up to two weeks within a fetal thymic (13-day) graft implanted into the anterior eye chamber of a syngenic host following its cervical sympathectomy (left side) and sham-operation (right side) 3-4 days earlier of implantation in mitogen, helper and effector cell assays. It is hoped that such a study will provide valuable information about the neuronal regulation of th immunogenesis of thymocytes and T lymphocytes.